Due to recent advances in the technology of laser generation and detection, laser systems for use in battlefield conditions have become more and more prevalent. These laser systems are employed for target illumination and tracking or for ranging. In a particular battlefield setting, there may be numerous laser illuminators operating simultaneously. These laser illuminators may be both from friendly forces and from enemy forces. In particular, combat troops operating in this environment will be subject to uncontrolled illumination by laser radiation. Because of the great radiated power from these laser radiation sources, these personnel require some manner of eye protection from this laser illumination. Furthermore, the modern battlefield also includes numerous optical sensors. Some of these sensors are associated with the above mentioned laser sources. Other sensors, such as those associated with infrared night vision systems, are independent of laser sources. These optical sensors likewise need protection from uncontrolled laser illumination.
Three types of laser protection are known in the prior art. The first type includes heavily tinted spectacles. The color of these tinted spectacles covers the bandwidth of the expected laser illumination. The laser light is absorbed by the tint in the spectacle, thereby reducing the light intensity reaching the eye within the wavelength band of the tint.
A second type of solution to this problem is the use of holographic optical elements. Holographic optical elements include three-dimensional interference fringe patterns which diffract light at specified wavelengths. Holographic optical elements are ordinarily constructed employing laser light forming interference fringes within the volume of a photosensitive medium. Upon development of the photosensitive medium, the pattern of interference fringes is fixed within this medium in form of varying indices of diffraction. When light of certain wavelengths enters such a holographic optical element, it is diffracted by this interference fringe pattern. In the case of laser protection eyewear, it is common to form a reflection holographic optical element which diffracts incoming radiation at the particular wavelength in a manner making it appear to be a mirror.
These laser eye protection techniques provide some protection for laser sources having known fixed wavelengths. In some instances it is easy to predict the particular wavelength to be employed because there are only a limited number of laser sources having a limited number of wavelengths. However, in the near future it is expected that multi-wavelength agile lasers will be employed in such battlefield situations. The advent of such multi-wavelength agile lasers complicates the strategies required to protect the eyes from laser radiation because of the increasing number or width of the attenuation bands required to cover all possible wavelengths will result in unacceptable degradation of visible transmission.
The third type of solution to the problem of laser protection is specifically directed to adaptive response to such multi-wavelength agile lasers. Several dynamic laser protection systems have been proposed which employ photodetector-actuated shutters which may be either mechanical, electro-optical or magneto-optical, or photoreactive filters such as formed of photochromatic materials. Such devices currently known are not completely satisfactory because they are cumbersome and they are slow in response. Known dynamic photo devices are not capable of achieving significant optical densities within 10 microseconds whereas the duration of a typical Q-switched laser pulse is approximately 20 nanoseconds. Therefore, currently known adaptive laser protection systems provide inadequate protection.
In view of the expected laser light environment in the modern battlefield a number of characteristics for laser protection would be advantageous. Firstly, this laser protection must be responsive to only the wavelengths of intense laser radiation. It should be able to respond extremely rapidly to any such intense laser radiation. The optical density, that is the blocking power, is ideally proportional to the intensity of the laser radiation. It would be advantageous that the attenuation bands of such laser protection in wavelength and angle be narrow enough to provide clear vision under all other conditions of illumination. Lastly, it would be advantageous if such a device were passive, that is not requiring any external power source.